I when on to a recycle website and asked for used garden solar lights, working or not.
After getting a response I went and picked up about 15 of these from a very nice person. Some lights worked about 1/2 did not. Mostly this was poor care.
These solar lights were of a higher price/quality type. You will notice each cell is upside down and has a switch to allow the LED to turn on or not. I'm pretty sure this is for storage.
I put the lights in the off position and left them off. For this project you will not need the lights and the solar cells will continue to charge the batteries.
I found the case I am using for this project at Harbor Freight for about $5-6 on sale.